1. Field of Invention
This invention relates to a single wavelength laser with grating-assisted dielectric waveguide coupler.
2. Brief Description of the Prior Art
Optical communication systems typically employ semiconductor laser sources and glass optical fiber communication channels. There are many configurations of semiconductor lasers including various material compositions and various dimensions of the grown layers that form the active region and an associated optical waveguide in the laser structure. The material composition of the active region determines the wavelength of operation. For example, at lasing wavelengths of about 0.9 .mu.m, the group III-V materials of the ternary compound Al.sub.x Ga.sub.(1-x) As with GaAs quantum wells provide a compact and rugged source of infrared light which can be easily modulated by varying the diode current. Communications systems of this type are discussed in Ser. No. 08/248,937, now issued as U.S. Pat. No. 6,064,783, the contents of which are incorporated herein by reference. Light from a laser can be extracted by abutting an optical fiber thereto in known manner, however, devices fabricated in this manner do not lend themselves to semiconductor fabrication. Lasers can be abutted to optical fibers, however the indices of refraction between optical fibers and semiconductor material are so dissimilar that the amount of coupling is very low, leading to an inefficient device. Furthermore, the alignment of the source with an optical fiber is quite tedious when high coupling efficiency is desired. This mismatch of the light field of the laser and that of the optical fiber also affects the amount of light coupled to the fiber.
In the device described in the above noted application, the direct coupling of the semiconductor laser output into an optical fiber was improved over the prior art by providing a semiconductor laser integrated with a silicon dioxide based waveguide having high efficiency coupling of the laser output into the waveguide by an integrated grating to permit the laser output to be coupled into an optical fiber by butt coupling of the optical fiber to the silicon dioxide based waveguide. The grating, when appropriately designed as discussed in the above noted application, provides a matching of the propagation in the laser with the propagation in the glass. The period of the grating determines the wavelength of that portion of the light in the laser waveguide that will be passed through the grating to the optical fiber. Multiple lasers with different wavelengths could be integrated and their outputs coupled and combined into a single waveguide for wavelength division multiplexed operation. A problem with the device of the above-mentioned application is that the narrow bandwidth of grating assisted directional couplers makes it difficult to match them with an integrated single wavelength laser source whose lasing wavelength must lie within the bandwidth of the coupler. The architecture of grating assisted directional couplers typically consists of two waveguides and an optical grating whose period is dictated by the geometry of the two waveguides. The geometrical properties include the waveguide dimensions as well as the refractive index profiles. Generally, the lasing wavelength is governed by an optical grating that produces the necessary feedback to the laser. Precise machining of both gratings must be made to allow for satisfactory operation.